The present invention relates generally to the field of wireless communications, and specifically to a method for discriminating between normal and truncated bursts in discontinuous transmission mode communications.
Discontinuous transmission (DTX) is a technique commonly used in wireless communications systems to reduce interference and conserve battery power. In conventional mobile communication networks, the mobile terminal transmits continuously on the uplink during a call. Normal conversation, however, contains a number of pauses between periods of speech, such as when a user listens to the other party. When a mobile terminal user is not speaking, transmission of the radio signal is not required from an information point of view. With discontinuous transmission, pauses in normal speech are detected in order to suspend radio transmission for the duration of the pause. Discontinuous transmission is typically an optional feature that can be enabled or disabled by the network as required. When used, discontinuous transmission can reduce air traffic, reduce interference between users, and extend battery life in mobile terminals.
During the periods during which no voice activity is detected, a transmitter may transmit nothing, or may transmit truncated bursts containing only radio control information. The transmission of at least some truncated bursts is desirable, to maintain a connection between the mobile terminal and the base station serving it, and to transmit control information. In either case, the state in which truncated bursts are transmitted is known as a DTX-low state; the state in which normal, full-length bursts are transmitted is known as a DTX-high state. The respective formats of a DTX-high, or normal, burst containing voice or data along with control information, and a DTX-low, or truncated, burst containing only control information, typically differ significantly. The wireless system receiver should be able to distinguish between the two, as the two types of burst transmissions are formatted differently, and the speech decoder will perform different operations based on whether the received data contains speech or random noise.
Several methods are known in the art for determining whether a received DTX burst is a normal burst or a truncated burst. Two examples of such methods are the use of Viterbi decoding metric and an estimated Bit Error Rate (BER) as the detection criteria. However, both methods suffer from poor DTX detection probability in high noise environments, i.e., when the Carrier-to-Interference Ratio (C/I) is low, or under high Rayleigh fading. At low C/I, both detection methods suffer from noise. For example, the Viterbi decoding metric can be an unreliable indicator if the correct path in the trellis corresponding to the correct code word is eliminated during channel decoding due to long bit error bursts in the received data. Similarly, the estimated BER is not necessarily the actual BER, as the receiver estimates the BER by re-encoding decoded data and then comparing the re-encoded data with the data originally received. Since the decoded data may have many residual bit errors after decoding, the re-encoded data may be similarly erroneous, and will result in incorrect BER estimates when compared with the received data. Statistically, the probability distributions of either detection method on normal and DTX truncated bursts overlap considerably, causing high detection error.